Seems relevant at this time to remind ourselves of the statistical meaning of correlation vs its popular use and perhaps more importantly why Ecology and Evolutionary Biology became and continue to be experimental sciences whenever possible.
>From the classic stats text Steele and Torrie (1980 p 277). "Correlation measures a co-relation, a joint property of two variables. Where variables are jointly affected because of external influences, correlation may offer the most logical approach to that analysis of the data. Regression deals primarily with the means of one variable and how their location changes with another variable. Š. Correlation is associated with descriptive techniques: regression has to do with a relation between population means and the values of a concomitant variable. Thus, whereas a correlation coefficient tells us something abut a joint relationship between variables, a regression coefficient tells us that if we alter the value of the independent variable then we can expect the dependent variable to alter by a certain amount on the average, sampling variation making it unlikely that precisely the stated amount of change will be observed." Thus, in Tom's example the correlation between churches and drunks implies not that either drives variation in the other, but simply that they covary, which may be a result of simple coincidence or that the are both responding to a common external driver. So, when most lay people talk about correlation, especially in looking for causal drivers, they are really implying regression and have a priori chosen one variable as the putative independent variable. Both approaches may IMPLY causation, regression by one of a pair of variables and correlation by some external driver affecting both variables, but neither can establish causation. Only well-designed experiments actually establish causation. These may identify causal factors phenomenologically (without necessarily identifying mechanism) or mechanistically, but either way are the only method for definitively establishing causal relationships. When used as the ultimate analysis (rather than for hypothesis generation) The elaborate and increasing sophisticated statistical methods of regression and elaborate models are quite simply a substitute for situations where experiments are infeasible. Good to never lose sight of that. William J. Resetarits, Jr Professor Department of Biological Sciences Texas Tech University Lubbock, Texas 79409-3131 Phone: (806) 742-2710, ext.300 Fax (806) 742-2963 On 10/9/12 8:01 PM, "Thomas J. Givnish" <givn...@facstaff.wisc.edu> wrote: >The number of drunks per city is very strongly correlated with the number >of churches per city. > >On 10/09/12, Lee Dyer wrote: >> My favorite *introduction* to this vast topic can be found in the first >>few chapters of Bill Shipley's short book, Cause and Correlation in >>Biology (2000). A quote from his book: >> "In fact, with few exceptions, correlation does imply >> causation. If we observe a systematic relationship between two >>variables, and >> we have ruled out the likelihood that this is simply due to a random >>coincidence, then something >> must be causing this relationship." >> >> ******************************************************* >> Lee Dyer >> Biology Dept. 0314 >> UNR 1664 N Virginia St >> Reno, NV 89557 >> >> >> >> OR >> >> >> >> 585 Robin St >> Reno, NV 89509 >> >> >> >> Email: nolaclim...@gmail.com >> Web: www.caterpillars.org >> phone: 504-220-9391 (cell) >> 775-784-1360 (office) >> >> >> >> >> > Date: Tue, 9 Oct 2012 10:57:34 -0500 >> > From: devan.mcgrana...@gmail.com >> > Subject: Re: [ECOLOG-L] correlation v. causation >> > To: ECOLOG-L@LISTSERV.UMD.EDU >> > >> > Hi Shelley, others, >> > >> > Slate recently had a great article on correlation and causation with a >> > historical perspective. >> > >> > My favorite line: "'No, correlation does not imply causation, but it >> > sure as hell provides a hint." >> > >> > >>http://www.slate.com/articles/health_and_science/science/2012/10/correlat >>ion_does_not_imply_causation_how_the_internet_fell_in_love_with_a_stats_c >>lass_clich_.html >> > >> > >> > >> > >> > >> > >> > >> > "Having nothing better to do, I set fire to the prairie." >> > -- Francis Chadron, 1839, Fort Clark, North Dakota >> > >> > http://www.devanmcgranahan.info > >-- > Thomas J. Givnish > Henry Allan Gleason Professor of Botany > University of Wisconsin > > givn...@wisc.edu > http://botany.wisc.edu/givnish/Givnish/Welcome.html
